Boat 454 mercruiser what does white smoke and white milky cream on the dipstick mean?

white milky cream = water in your oil.

An engine used frequently heats the oil driving off what little moisture finds it way into the crankcase. Water is found in the oil when the boat is left idle for a long time. At this time of year a lot of people are just putting the boat in the water after storing it all winter. If the engine was oil fogged to prevent corrosion while in storage it would produce copious white smoke when started the first time.

If it has been used regularly, then a gasket may have failed, or there is leak in the block or head.